Fishes
are vertebrates, with internal skeletons
that grow inside the animal.

Fishes
have a body pattern
that has proved to be
very
successful.
The backbone provides
flexibility of
movement, and a base
from which ribs can
grow to protect the
internal organs. Limb
bones can be organized
around the spine.

In
fishes the skeleton
has ossified, which
means that the bones
are hard and made of
calcium
carbonate. This
gives the internal
skeleton
strength.
However, fish do not
need their bones to be
strong enough to
support their bodies:
the water supports
them.

the head contains the brain, eyes, internal ears, nasal sacs,
and mouth

breathing begins in the mouth, and oxygen is extracted by
the gills

the skeleton has a bony head that protects the brain, a backbone,
and bones that support the body

the digestive process begins in the mouth and goes through
stomach and intestines to end near the back of the body

the
circulatory system has a two chambered heart and blood vessels
that feed the body's cells, bring them oxygen, and carrying away
wastes

there are two sexes: females produce eggs, males produce
sperm. (However, in some species, males later turn into females
or females turn into males.) Most fishes lay their eggs in the
water, but a few, such as guppies, are live bearers.

the pectoral (front) fins stabilize the body and help it
to turn: later they become front legs, arms, or wings in other
kinds of animals

the pelvic (back) fins also stabilized the body: they are
the forebears of back legs

the tail provides propulsive power and helps with balance

extensions of the spine support the dorsal fin (the fin on
the back) which provides balance. (Similar projections show up
in the dinosaurs and mammals such as the buffalo, although their
function has changed.)

Some fish have swim bladders to help them with buoyancy.

Fishes and Buoyancy

Fishes have two problems
to solve: one is physical, the problem of moving in the water:
the other is chemical, the problem of keeping their body fluids
at the right balance of salinity (saltiness).

Most fishes have body shapes that move easily in water. The
fins help to keep the body right side up. Fishes need to be buoyant:
if their bodies were too heavy they would lie on the bottom of
the lake or sea and would have to work hard to swim. Fish need
to weigh the same as, or a little less than, the water that would
fill the space occupied by their bodies. They do not have massive
(heavy) bones, and some have a swim bladder, an organ that is
filled with a gas and so makes the body
mass lighter in relationship
to the water around it.

Fishes and Salinity

Life began in the early oceans, and the cells developed to
work best with a certain amount of salt in their protoplasm.
However, as time went on, salt continued to build up in the oceans,
and so ocean fishes have less salt in their tissues than the
water that surrounds them. They drink a lot of water, and have
developed ways to get rid of the extra salt in their bodies.

Some fishes live in rivers and lakes. Their body tissues are
more salty than the water around them. They drink only a little
of the water to conserve their salt.

Fishes and Temperature

Fishes are cold blooded, which means that their bodies are
at the same temperature as the surrounding water. Each species
of fish has a temperature range in which it functions well. If
the water gets too cold, the fish almost stops eating and just
waits. This adaptation allows it to survive over winters unless
the temperature gets too low or remains too low too long, in
which case it dies. If the temperature gets too high, the fishes
die.

Fish are able to sense temperatures which are appropriate
for them. They seek out these temperatures: for instance, in
a lake they may seek out shallow water which has been warmed
by the sun or deep water in the middle of the lake, which is
cooler.

Life processes are chemical, and chemical reactions take place
more rapidly as temperature rises. Would you expect fish to grow
more rapidly in warm or cold water?

Fishes and Adaptation

Fishes
have been on earth
since Devonian times,
and are among the most
successful of earth's multicellular
life forms. They have had hundreds of millions of years to adapt
to living in aquatic communities. They have developed into many
species. Fishes of many different sizes, body shapes, and colors
inhabit the waters of our planet.